1. Field of Invention
This invention relates generally to optical information storage systems for the storage and retrieval of information and, more particularly, to an information storage medium for use in an optical storage system making use of magneto-optical materials for the storage of information.
2. Description of the Related Art
The optical information storage systems generally referred to as magneto-optical storage systems, are of particular interest because of the ability to alter or over-write information already stored on the medium. In these systems, a magnetic medium exhibiting the Kerr effect is used to store information. The Kerr effect provides that magnetic materials having a component of magnetic orientation parallel to a direction of impinging radiation will affect the (linear) polarization of reflected radiation depending upon whether the magnetic orientation is parallel or anti-parallel to the direction of impinging radiation. Although the difference in the rotation of the plane of polarization is typically less than 2.degree., this difference has been found to be sufficient to provide a basis for an optical storage system. In this storage system, the orientation of magnetic material in one direction represents a first logical state while the orientation in the opposite direction represents a second logical state.
The materials used in the magneto-optical storage systems are typically rare earth/transition metal alloy thin films. Examples of the materials currently used in the magneto-optical storage systems include Gadolinium-Cobalt thin films, Gadolinium-Iron thin films, Terbium-Iron thin films, Dysprosium-Iron thin films, etc. The magneto-optical storage systems have several complexities in implementation, such as the requirement for an external field during the storage medium write cycle, which has been the subject of extensive development activity. By way of specific example, U.S. Pat. No. 4,853,912, issued to Akasaka et al., discloses the positioning of a magnetic material along the sides of the information paths or tracks. This provides the bias field. U.S. Pat. No. 4,794,560 issued to Bell et al. and Japanese Patent 62-154347 issued to Horigome, are examples of storage media in which a plurality of layers of magnetic material are used to provide bias conditions. U.S. Pat. No. 4,628,485 issued to Tanaka et al. describes a method of improving the Kerr rotation. These patents are indicative of efforts to provide a more convenient write storage medium.
While the effort to develop a convenient storage material for writing or overwriting of information has received the principal attention, a continuing concern in the magneto-optical recording systems is the limitation on the density of information that can be effectively read by the system. In particular, the limitation on the storage density is typically a result of the size of the radiation beam applied to the surface of the magneto-optical storage medium. In the article by Aratani et al. entitled "Magnetically Induced Super Resolution In Novel Magneto-Optic Disk", S.P.I.E. Proceedings, vol. 1499. page 209 (1991), a system for providing an optical storage medium capable of storing a higher density of information is disclosed. However, the disclosed system requires an initializing field.
A need has been felt for materials and an associated technique for reading information in a magneto-optical storage system which is not density-limited by the size of the area of illumination of the radiation in reading the stored information and which does not require a initialization field for the reading operation.